1. Field of the Invention
Generally, the present disclosure relates to the manufacture of sophisticated semiconductor devices, and, more specifically, to various methods of forming a metal cap layer on copper-based conductive structures on integrated circuit devices and integrated circuit devices having such a structure.
2. Description of the Related Art
The fabrication of advanced integrated circuits, such as CPU's, storage devices, ASIC's (application specific integrated circuits) and the like, requires the formation of a large number of circuit elements in a given chip area according to a specified circuit layout. Field effect transistors (NFET and PFET transistors) represent one important type of circuit element that substantially determines the performance capabilities of the integrated circuits. During the fabrication of complex integrated circuits using, for instance, MOS technology, millions of transistors, e.g., NFET transistors and/or PFET transistors, are formed on a substrate including a crystalline semiconductor layer. A basic field effect transistor comprises a source region, a drain region and a channel region extending between the source and drain regions. Such a transistor further includes a gate insulation layer positioned above the channel region and a gate electrode positioned above the gate insulation layer. When an appropriate voltage is applied to the gate electrode, i.e., a voltage that exceeds the threshold voltage of the transistor, the channel region becomes conductive and current may flow from the source region to the drain region. The gate electrode may be made of a variety of materials, e.g., polysilicon, one or more layers of metal or combinations thereof. The gate structure of the transistor may be made using so-called “gate-first” or “replacement gate” techniques. In one embodiment, the basic structure of a field effect transistor is typically formed by forming various layers of material and thereafter patterning those layers of material using known photolithography and etching processes. Various doped regions, e.g., source regions, drain regions, halo regions, etc., are typically formed by performing one or more ion implantation processes through a patterned mask layer using an appropriate dopant material, e.g., an N-type dopant or a P-type dopant, to implant the desired dopant material into the substrate. The particular dopant selected depends on the specific implant region being formed and the type of device under construction, i.e., an NFET transistor or a PFET transistor. During the fabrication of complex integrated circuits, millions of transistors, e.g., NFET transistors and/or PFET transistors, are formed on a substrate by performing a number of process operations.
However, the ongoing shrinkage of feature sizes on transistor devices causes certain problems that may at least partially offset the advantages that may be obtained by reduction of the device features. Generally, decreasing the size of, for instance, the channel length of a transistor typically results in higher drive current capabilities and enhanced switching speeds. Upon decreasing the channel length, however, the pitch between adjacent transistors likewise decreases, thereby limiting the size of the conductive contact elements—e.g., those elements that provide electrical connection to the transistor, such as contact vias and the like—that may fit within the available real estate between adjacent transistors. Accordingly, the electrical resistance of conductive contact elements becomes a significant issue in the overall transistor design, since the cross-sectional area of these elements is correspondingly decreased. Moreover, the cross-sectional area of the contact vias, together with the characteristics of the materials they comprise, may have a significant influence on the effective electrical resistance and overall performance of these circuit elements.
A typical integrated circuit product will typically comprise multiple metallization layers, e.g., 10-14 metallization layers. In general, the metallization layers are comprised of layers of insulating material having various conductive metal lines and vias formed therein. In effect, the conductive structures in these various metallization layers constitute the “wiring” arrangement for the various elements of the electrical circuit, e.g., transistors, resistors, capacitors, etc., that are formed in a semiconducting substrate. Increasing the functionality and performance capability of various metallization systems has become important in designing modern semiconductor devices. One example of such improvements is the enhanced use of copper metallization systems in integrated circuit devices and the use of so-called “low-k” dielectric materials (materials having a dielectric constant less than 3) in such devices. Copper metallization systems exhibit improved electrical conductivity as compared to, for example, prior art metallization systems using aluminum for the conductive lines and vias. The use of low-k dielectric materials also tends to improve the signal-to-noise ratio (S/N ratio) by reducing crosstalk as compared to other dielectric materials with higher dielectric constants. However, the use of such low-k dielectric materials can be problematic as they tend to be less resistant to metal migration as compared to some other dielectric materials.
Copper is a material that is difficult to etch using traditional masking and etching techniques. Thus, conductive copper structures, e.g., conductive lines or vias, in modern integrated circuit devices are typically formed using known single or dual damascene techniques. In general, the damascene technique involves: (1) forming a trench/via in a layer of insulating material; (2) depositing one or more relatively thin barrier layers; (3) forming copper material across the substrate and in the trench/via; and (4) performing a chemical mechanical polishing process to remove the excess portions of the copper material and the barrier layer positioned outside of the trench/via to define the final conductive copper structure. The copper material is typically formed by performing an electrochemical copper deposition process after a thin conductive copper seed layer is deposited by physical vapor deposition on the barrier layer.
In many cases, a metal cap layer is formed above the copper structures to protect the underlying copper structures. One technique employed in an attempt to form such metal cap layers involved doping the copper seed layer with a metal, such as copper or manganese. During a high temperature anneal, the dopants were to segregate to the top of the copper structure, i.e., to the top of the metal line. However, given overall device scaling, the thickness of the copper seed layers is so small that a sufficient quantity of dopant material cannot be introduced into the copper seed layer so as to form an effective metal cap layer. Selective deposition processes have been used to selectively deposit a metal cap layer directly on only the copper structure. However, such selective deposition processes are extremely slow, thereby reducing overall product throughput. Moreover, such selective deposition processes typically require pre-deposition cleaning processes and post-deposition plasma or thermal anneal treatments, all of which may degrade device reliability.
The present disclosure is directed to various methods and devices that may avoid, or at least reduce, the effects of one or more of the problems identified above.